This sounds a bit familiar. I struggled to configure googlemail to collect email via SMTP. In the end I discovered I needed to configure it to use the bigv.io url, rather than use the domain name, due to a certificate error as unencrypted mail was not allowed.

Outgoing

Fogma - you're right all other ports work, including 587. Although I don't need to use the bigv.io url.

Adhawkins - the firewall blacklist is based on IP addresses and not ports, although it scans for malicious activity accessing SMTP, it doesn't block the port. Turns out the IP addresses I'd seen hadn't been automatically blocked

Port 465 has never been standardised as an email service. It was required by some Microsoft mail clients, but modern Microsoft clients use 587 by default now. And port 587 is standardised as a mail submission port.